Memory devices are typically provided as internal storage areas in the computer. The term memory identifies data storage that comes in the form of integrated circuit chips. There are several different types of memory used in modern electronics, one common type is RAM (random-access memory). RAM is characteristically found in use as main memory in a computer environment. RAM refers to read and write memory; that is, you can both write data into RAM and read data from RAM. This is in contrast to ROM, which permits you only to read data. Most RAM is volatile, which means that it requires a steady flow of electricity to maintain its contents. As soon as the power is turned off, whatever data was in RAM is lost.
Computers almost always contain a small amount of read-only memory (ROM) that holds instructions for starting up the computer. Unlike RAM, ROM cannot be written to. An EEPROM (electrically erasable programmable read-only memory) is a special type non-volatile ROM that can be erased by exposing it to an electrical charge. EEPROM comprise a large number of memory cells having electrically isolated gates (floating gates). Data is stored in the memory cells in the form of charge on the floating gates. Charge is transported to or removed from the floating gates by specialized programming and erase operations, respectively.
Yet another type of non-volatile memory is a Flash memory. A Flash memory is a type of EEPROM that can be erased and reprogrammed in blocks instead of one byte at a time. A typical Flash memory comprises a memory array, which includes a large number of memory cells. Each of the memory cells includes a floating gate field-effect transistor capable of holding a charge. The data in a cell is determined by the presence or absence of the charge in the floating gate. The cells are usually grouped into sections called “erase blocks.” The memory cells of a Flash memory array are typically arranged into a “NOR” architecture (the cells arranged in an array of rows and columns, each cell directly coupled to a bitline) or a “NAND” architecture (cells coupled into “strings” of cells, such that each cell is coupled indirectly to a bitline and requires activating the other cells of the string for access). Each of the cells within an erase block can be electrically programmed in a random basis by charging the floating gate. The charge can be removed from the floating gate by a block erase operation, wherein all floating gate memory cells in the erase block are erased in a single operation.
Because all the cells in an erase block of a Flash memory device must be erased all at once, one cannot directly rewrite a Flash memory cell without first engaging in a block erase operation. Erase block management (EBM) provides an abstraction layer for this to the host, allowing the Flash device to appear as a freely rewrite-able device. Erase block management also allows for load leveling of the internal floating gate memory cells to help prevent write fatigue failure. Write fatigue is where the floating gate memory cell, after repetitive writes and erasures, no longer properly erases and removes charge from the floating gate. Load leveling procedures increase the mean time between failure of the erase block and Flash memory device as a whole.
In many modern Flash memory devices implementations, the host interface and/or erase block management routines additionally allow the Flash memory device to appear as a read/write mass storage device (i.e., a magnetic disk) to the host, storing data in the Flash memory in 512-byte sectors. As stated above, the erase block management routines along with the address translation layer provide the necessary linkage between the host and the internal Flash memory device erase block array; logically mapping logical sectors to physical sectors on the Flash device and managing block erasure.
To accomplish this mapping of a logical sector to a physical sector in the Flash memory of the prior art, either a table is kept in RAM or the physical sectors are scanned for the physical sector that contains the requested logical sector address. With the data storage capacity of modern Flash memories increasing issues are being caused with the size of the required RAM table and/or the time required to scan the Flash memory for the requested sector. This is particularly an important issue in resource limited handheld or embedded devices.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for a Flash memory device and/or erase block management with improved logical to physical sector mapping.